Processes involving catalytic hydrogenation of carboxylic acids or esters thereof having an aldehyde or keto group are known in the art. The hydrogenation of ethyl pyruvate over an alumina-supported platinum catalyst, for example, is described in J. Molecular Catalysis, 49(2), 195-203 (1989).
Levulinic acid can be catalytically hydrogenated to give the corresponding gamma-hydroxy acid 4-hydroxypentanoic acid (known as hydroxyvaleric acid) which subsequently cyclises to form the 5-membered lactone, gammavalerolactone. Early studies reported the hydrogenation of levulinic acid to gammavalerolactone using an unsupported platinum oxide catalyst (J. Am. Chem. Soc, 52, 3010 (1931)).
Other studies have also been reported. In U.S. Pat. No. 5,883,266, for example, the preparation of gammavalerolactone from levulinic acid is described by hydrogenation with a bifunctional metallic catalyst comprising a noble metal, copper, nickel or rhenium, or combinations thereof, optionally on a support which is preferably carbon but which may also be a metal oxide such as alumina or magnesium silicate or combinations thereof. Titania and zirconia supports are mentioned but not exemplified. A preferred catalyst is disclosed to be palladium-rhenium catalyst supported on a carbon support.
The catalytic hydrogenation of levulinic acid to give gammavalerolactone using a metal selected from Group VIII of the Periodic Table, optionally on a catalyst support is also described in WO 02/074760. It is described that a preferred metal catalyst is ruthenium; the support is preferably carbon, SiO2 and Al2O3, with oxidatively stable carbon being particularly preferred.
Hydrogenation of levulinic acid or its esters to give gammavalerolactone using a hydrogenating metal catalyst on a support is also described in WO 2006/067171. Metal catalysts which are specifically exemplified include rhodium, ruthenium, palladium, nickel and nickel/platinum and rhenium/platinum combinations on a zeolite/silica or silica support.
A significant problem associated with such hydrogenation processes is the susceptibility of the catalyst to poisoning and/or degradation by the acid reactant. Both the transition metal and basic materials in the catalyst support can potentially react with carboxylic acids causing the metal or support to leach or dissolve. Carbon supports overcome the problem of leaching but do not allow for the regeneration of deactivated catalyst by coke burn-off. There therefore remains a need for improved catalyst systems for use in such processes.
Levulinic acid is readily available from cellulose feedstock material and is therefore a convenient starting material for the preparation of gammavalerolactone. Products such as ethyl valerate and pentyl valerate which are obtainable from gammavalerolactone by hydrogenation are of particular interest as fuel components and there therefore remains a particular continuing interest in the development of improved methods for preparing gammavalerolactone from levulinic acid.